This invention relates to a vapor filter with partial bypass.
A forced air central air conditioning system (which term, as used herein, includes a heating and/or cooling system) of conventional construction comprises a plenum having a downstream end from which air distribution ducts extend, an upstream end for receiving return air, a heat exchanger disposed between the upstream end of the plenum and the downstream end thereof, and a blower between the upstream end of the plenum and the heat exchanger for establishing a flow of air from the upstream end of the plenum over the heat exchanger to the downstream end of the plenum.
It is usual for a particulate filter device to be mounted just upstream of the blower in order to protect the blower and heat exchanger from accumulation of dust and other airborne particles, which would impair the efficiency of the blower and heat exchanger and might even give rise to a risk of fire.
A figure of merit for the particulate filter device that is installed in a domestic forced air central air conditioning system is the ASHRAE efficiency designation. The ASHRAE efficiency designation for filter devices that are commonly sold for domestic use is often as low as 5%. Even with such a low ASHRAE efficiency designation, these filter devices do an adequate job of protecting the blower and heat exchanger from particulate accumulation. A low ASHRAE efficiency designation has the advantage that the filter device does not present a high flow resistance, which would result in a high load being imposed on the blower.
The typical filter device installed in a domestic forced air central air conditioning system comprises a frame of cardboard and a web of porous material supported in the frame. The filter device may also comprise a support web to retain the filter web in position against the force applied to the filter web during operation of the blower.
It has in recent years been recognized that indoor air pollution can have significant health consequences. Most indoor air pollutants can be classified either as particles or as vapors. The conventional filter device of a domestic forced air central air conditioning system, having an ASHRAE efficiency designation of about 5%, does an acceptable job in removing fairly large particles, such as household dust, but it does not retain smaller particles, such as those present in cigarette smoke, or vapors.
It is known to use activated carbon as a vapor filter. In principle, it would be possible to place a bed of activated carbon over the filter web of a filter device for use in a domestic forced air central air conditioning system, but the flow resistance of the modified filter device would likely place an unacceptable load on the blower. It has therefore been proposed to employ a partial bypass filter, in which some of the air passing through the filter device contacts the activated carbon and some bypasses the activated carbon.
It is known to construct an activated carbon filter with a partial bypass by using tubes of perforated sheet metal filled with activated carbon and arranged in spaced parallel relationship across an air flow duct. Some of the air passes between the tubes, while other air passes through the perforations in the walls of the tube and is exposed to the activated carbon inside the tube. This type of filter is costly to manufacture because the perforated sheet metal is expensive.
Typically, the mounting structure for the particulate filter device installed in a domestic forced air central air conditioning system will accommodate a rectangular filter device having a thickness no greater than about 2.5 cm. In order to install a vapor filter with partial bypass in such a system without modifying the mounting structure, the vapor filter device must have substantially the same overall dimensions as the conventional particle filter.